The present invention relates to an aqueous silicone coating composition having an excellent pot life; and to a polyester release film coated by said composition on at least one surface thereof, which is used as a temporary support substrate for an adhesive coated article.
Polyesters such as polyethylene terephthalate (PET) are known to possess good chemical stability, physical and mechanical strength, heat resistance, durability, chemical resistance, weather resistance and electrical insulation property; and, therefore, have been widely used in manufacturing various articles including medical devices, capacitors, photographic films, packing and labeling materials, and magnetic recording media.
In addition, a silicone-coated polyester film may be used as a release substrate for adhesive coated articles such as adhesive tapes and peel-off label stickers. A silicone-coated polyester film having high strength, transparency and chemical resistance is particularly desirable in such applications.
Silicone-coated release films are produced by coating a substrate film with a solvent-based or solventless silicone composition. The use of a solvent-based silicone composition, however, induces the problem of releasing harmful organic solvents to the environment, while the coating obtained using a solventless silicone composition which usually has a high viscosity is undesirably thick, which leads to economic disadvantages.
Accordingly, there have been a number of studies to develop aqueous silicone coating compositions.
Canadian Patent No. 1,120,176 discloses an aqueous silicone emulsion consisting of: 1) a diorganopolysiloxane having silicon bonded vinyl groups in their terminal units, 2) an organopolysiloxane having at least three silicon-bonded hydrogen atoms per molecule, and 3) a catalytic amount of a platinum catalyst which promotes the addition of the silicon-hydrogen bonds to the vinyl groups. Similar aqueous-based silicone compositions containing polyvinyl alcohol and alkylphenyl polyglycol ether as emulsifying agents are disclosed in Canadian Patent No. 1,246,270, European Patent Publication No. 484,001, U.S. Pat. Nos. 5,095,067; 5,104,927; 4,791,029; 3,900,617; and 5,777,017.
However, release films coated with the aqueous silicone emulsions mentioned above have poor chemical resistance and unsatisfactory storage stability.
U.S. Pat. No. 5,672,428 teaches an aqueous silicone coating composition comprising: 1) an alkylvinylpolysiloxane containing vinyl groups, 2) a platinum or tin complex catalyst, 3) a glycidoxysilane, and 4) an alkylhydrogenpolysiloxane. Release films coated with this silicone composition exhibit improved chemical and rub-off resistances, but the productivity of this method is not good because the pot life of the composition is severely reduced.
Accordingly, it is an object of the present invention to provide a silicone coating composition having a good pot life, which is capable of forming a coating having good release property, improved rub-off and chemical resistances.
It is another object of the present invention to provide a polyester release film of which at least one surface is coated with the composition.
In accordance with one aspect of the present invention, there is provided an aqueous silicone coating composition comprising an alkylvinylpolysiloxane; 1.5 to 10 percent by weight, based on the amount of the alkylvinylpolysiloxane, of an alkylhydrogenpolysiloxane; 10 to 500 ppm, based on the amount of the alkylvinylpolysiloxane, of a platinum complex catalyst; and 0.5 to 20 percent by weight, based oil the amount of the alkylvinylpolysiloxane, of a water-soluble polyester resin.
In accordance with the present invention, there is provided an aqueous silicone coating composition comprising an alkylvinylpolysiloxane, an alkylhydrogenpolysiloxane, a platinum complex catalyst and a water-soluble polyester resin.
The alkylvinylpolysiloxane which may be used in the present invention is a straight- or branched-chain type, wherein preferred is the straight-chain type. The straight-chain type alkylvinylpolysiloxane has two or more of R2 groups per molecule on the average and a mean viscosity of 10 to 30,000 mm2/sec, preferably 200 to 7,000 mm2/sec at 25xc2x0 C. Divinylpolydimethylsiloxane having two vinyl groups in its terminal units is most preferably employed in the present invention.
The alkylhydrogenpolysiloxane employed in the present invention is a straight-chain, branched-chain or ring type, wherein preferred is the straight-chain type. The straight-chain type alkylhydrogenpolysiloxane has two or more silicon-bonded hydrogen atoms per molecule on the average and a mean viscosity of 10 to 1,000 mm2/sec, preferably 10 to 100 mm2/sec at 25xc2x0 C. Polymethylhydrogensiloxane having a hydrogen content of at least 1 weight % is most preferably employed in the present invention. The alkylhydrogenpolysiloxane may be employed in an amount ranging from 1.5 to 10 parts, preferably 2 to 5 parts by weight based on 100 parts by weight of the alkylvinylpolysiloxane.
The alkylvinylpolysiloxane and alkylhydrogenpolysiloxane are cross-linked and cured by inducing a hydrosilation reaction therebetween, the hydrosilation reaction being accelerated by a platinum catalyst such as platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex. The platinum complex catalyst may be employed in an amount ranging from 10 to 500 ppm based on the weight of the alkylvinylpolysiloxane.
Aqueous silicone emulsions comprising such alkylvinylpolysiloxanes, alkylhydrogenpolysiloxanes and platinum catalysts are commercially available, for example from Dow Corning(Midland, Mich.), GE Silicone(Waterford, N.Y.) and Rhone Poulenc, Inc.(Rockwell, S.C.) under the trade names of Syl-Off 7900/7922, SM 3000/3010 and 71822/71823, respectively. These emulsions are composed of two separate emulsions, i.e., a first emulsion containing an alkylvinylpolysiloxane and an alkylhydrogenpolysiloxane, and a second emulsion, an alkylvinylpolysiloxane and a platinum catalyst.
The water-soluble polyester resin employed in the present invention is water soluble or water dispersible, and contains a controlled amount of ester bonds formed between a dicarboxylic acid having hydrophilic groups and a diol in its main chain.
The amount of the dicarboxylic acid units having hydrophilic groups is preferably below 15 mol %, more preferably 5 to 15 mol %, based on the total dicarboxylic acid units present in the water-soluble polyester resin, wherein examples of hydrophilic groups are alkali metal sulfonate and alkali metal carboxylate groups. Other dicarboxylic acid units are derived from such conventional dicarboxylic acids as dimethylterephthalic acid, terephthalic acid, dimethylisophthalic acid, isophthalic acid, naphthalenedicarboxylic acid, cyclohexanedicarboxylic acid, diphenoxyethanedicarboxylic acid, biphenyldicarboxylic acid, diphenyl ether dicarboxylic acid, anthracenedicarboxylic acid and xcex1, xcex2-bis(2-chlorophenoxy)ethane-4,4xe2x80x2-dicarboxylic acid, among which a combination of dimethylterephthalic acid or terephthalic acid and isophthalic acid(50xcx9c80:20xcx9c50(mol %)) is preferred.
Examplary diols which are used in preparing the water-soluble polyester resin include: ethylene glycol, 1,4-butanediol, propylene glycol, neopentyl glycol, 1,6-hexanediol, diethylene glycol, triethylene glycol, polyethylene glycol and polypropylene glycol, among which a combination of ethylene glycol and 1,4-butanediol is preferred.
The water-soluble polyester resin of the present invention has a number average molecular weight of less than 50,000, preferably below 25,000, and it may be employed in an amount ranging from 0.5 to 20 parts, preferably 1 to 10 parts by weight based on 100 parts by weight of the alkylvinylpolysiloxane. When the amount of the water-soluble resin used is less than 0.5 parts by weight, the strength of coated film becomes unsatisfactory, while at an amount of more than 20 parts by weight, the force required for peeling-off becomes too great.
Such water-soluble polyester resins are commercially available, for example from SK Chemical Inc.(Korea) and Whaseong Inc.(Japan) under the trade names of SK-100 and SK-300, and WR-901, respectively.
The silicone coating compositon of the present invention may further comprise a curing inhibitor(e.g., 1-ethynylcyclohexanol) to prevent premature cure of the silicon resins at room temperature, and an emulsifying agent(e.g., a combination of polyvinyl alcohol or polyethylene and alkyl phenyl polyglycol ether).
The water content of the aqueous coating composition of the present invention may be in the range of 70 to 97%, preferably 85 to 95% by weight based on the total weight of the composition.
In accordance with another aspect of the present invention, there is provided a polyester release film having a coating formed using the inventive silicone composition, on at least one surface thereof.
The polyester substrate film used in the present invention is made of a conventional polyester resin produced by transesterifying and polycondensing a dialkyl ester of an aromatic dicarboxylic acid and an aliphatic glycol.
Representative dialkyl esters of aromatic dicarboxylic acids include: dialkyl esters of terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, cyclohexanedicarboxylic acid, diphenoxyethanedicarboxylic acid, biphenyldicarboxylic acid, diphenyl ether dicarboxylic acid, anthracenedicarboxylic acid and xcex1, xcex2-bis(2-chlorophenoxy)ethane-4,4xe2x80x2-dicarboxylic acid, among which dimethyl terephthalate and terephthalic acid are preferred.
Examplary aliphatic glycols include: ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol and hexylene glycol, wherein preferred is ethylene glycol.
A polyester which may be preferably used in the present invention is composed of 60 mol % or more of ethylene terephthalate repeat units, the remainder being copolymeric repeat units derived from dicarboxylic acids or multi-functional dicarboxylic acids and other diols. Such dicarboxylic acids and multi-functional dicarboxylic acids include: adipic acid, sodium 5-sulfoisophthalate, trimellitic acid and pyrromellitic acid; and said other diols include: diethylene glycol, propylene glycol, neopentyl glycol, polyethylene glycol, p-xylene glycol, 1,4-cyclohexane dimethanol and sodium 5-sulforesolcine.
A polyester film may be prepared by employing a conventional method, for example, by melt-extruding a polyester resin into a cast sheet through a T-die, quenching the sheet on a cold roll, drawing the resulting sheet biaxially and then heat-setting the biaxially oriented film at a temperature ranging from 190 to 240xc2x0 C.
In accordance with the present invention, the inventive aqueous silicone coating composition is applied to at least one surface of a polyester substrate film and dried to give a polyester release film.
The inventive polyester release film may be obtained by xe2x80x9cin-line coatingxe2x80x9d or xe2x80x9coff-line coatingxe2x80x9d a surface of a substrate film with the inventive composition using any appropriate method selected from roller coating, spray coating, gravure coating, reverse gravure coating, myer bar coating and die coating. Off-line coating is carried out after the heat-setting of a substrate film, and in-line coating, typically between the longitudinal stretching and heat-setting stages of the polyester film manufacturing process. The in-line coating method is preferred because a fully dried coating is obtained.
The thickness of the coating of the inventive polyester release film is 50 to 300 nm.